Overview

Brief Summary

The species that you and all other living human beings on this planet belong to is Homo sapiens. During a time of dramatic climate change 200,000 years ago, Homo sapiens (modern humans) evolved in Africa. Like other early humans that were living at this time, they gathered and hunted food, and evolved behaviors that helped them respond to the challenges of survival in unstable environments.

Anatomically, modern humans can generally be characterized by the lighter build of their skeletons compared to earlier humans. Modern humans have very large brains, which vary in size from population to population and between males and females, but the average size is approximately 1300 cubic centimeters. Housing this big brain involved the reorganization of the skull into what is thought of as "modern" -- a thin-walled, high vaulted skull with a flat and near vertical forehead. Modern human faces also show much less (if any) of the heavy brow ridges and prognathism of other early humans. Our jaws are also less heavily developed, with smaller teeth.

Scientists sometimes use the term “anatomically modern Homo sapiens” to refer to members of our own species who lived during prehistoric times.

The most salient physiological development between the earlier australopith species and Homo is the increase in cranial capacity, from about 450 cm3 (27 cu in) in A. garhi to 600 cm3 (37 cu in) in H. habilis. Within the Homo genus, cranial capacity again doubled from H. habilis through Homo ergaster or H. erectus to Homo heidelbergensis by 0.6 million years ago. The cranial capacity of H. heidelbergensis overlaps with the range found in modern humans.

The advent of Homo was thought to coincide with the first evidence of stone tools (the Oldowan industry), and thus by definition with the beginning of the Lower Palaeolithic; however, recent evidence from Ethiopia now places the earliest evidence of stone tool usage at before 3.39 million years ago.[5] The emergence of Homo coincides roughly with the onset of Quaternary glaciation, the beginning of the current ice age.

Homo sapiens (modern humans) is the only surviving species in the genus, all others having become extinct. Homo neanderthalensis, traditionally considered the last surviving relative, died out about 24,000 years ago, though recent discoveries suggest that another species, Homo floresiensis, may have lived much more recently. The other extant Homininae—the chimpanzees and gorillas—have a limited geographic range. In contrast, the evolution of humans is a history of migrations and admixture. Humans repeatedly left Africa to populate Eurasia and finally the Americas, Oceania, and the rest of the world.

The word homo is Latin, in the original sense of "human being", or "man" (in the gender-neutral sense). The word "human" itself is from Latin humanus, an adjective cognate to homo, both thought to derive from a Proto-Indo-European word for "earth" reconstructed as *dhǵhem-.[6]

Names for other species were coined beginning in the second half of the 19th century (H. neanderthalensis 1864, H. erectus 1892). A couple of recently discovered, recently extinct, species in the Homo Genus do not have accepted binomial names yet, Denisova hominin, and Red Deer Cave people. Classification of the Homo Genus into species and subspecies is poorly defined, highly disputed, and subject to political correctness and incomplete information, leading to difficulties in binomial naming, and the use of common names such as Neanderthal and Denisovan even in scientific papers.[9]

H. habilis, which is considered the first member of the genus Homo, might have given rise to H. ergaster (however questionable, as some finds suggest both species were contemporaneous).[11] Some of H. ergaster migrated to Asia, where they are named Homo erectus, and to Europe with Homo georgicus. H. ergaster in Africa and H. erectus in Eurasia evolved separately for almost two million years and presumably separated into two different species. Homo rhodesiensis, who were descended from H. ergaster, migrated from Africa to Europe and became Homo heidelbergensis and later (about 250,000 years ago) Homo neanderthalensis and the Denisova hominin in Asia. The first Homo sapiens, descendants of H. rhodesiensis, appeared in Africa about 250,000 years ago. About 100,000 years ago, some H. sapiens sapiens migrated from Africa to the Levant and met with resident Neanderthals, with some admixture.[12] Later, about 70,000 years ago, perhaps after the Toba catastrophe, a small group left the Levant to populate Eurasia, Australia and later the Americas. A subgroup among them met the Denisovans[13] and, after further admixture, migrated to populate Melanesia. In this scenario, non-African people living today are mostly of African origin ("Out of Africa model"). However, there was also some admixture with Neanderthals and Denisovans, who had evolved locally (the "multiregional hypothesis"). Recent genomic results from the group of Svante Pääbo also show that 30,000 years ago at least three major subspecies coexisted: Denisovans, Neanderthals and anatomically modern humans.[14] Today, only H. sapiens remains, with no other extant species.

^dhghem The American Heritage Dictionary of the English Language: Fourth Edition. 2000.

^Note: In 1959, Linnaeus was designated as the lectotype for Homo sapiens (Stearn, W. T. 1959. "The background of Linnaeus's contributions to the nomenclature and methods of systematic biology", Systematic Zoology 8 (1): 4-22, p. 4) which means that following the nomenclatural rules, Homo sapiens was validly defined as the animal species to which Linnaeus belonged.

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Denisova hominin

The Denisova hominin (/dɨˈnɪsəvə/) is the name given to the remains of a member of the genus Homo that may be a previously unknown species based on an analysis of its mitochondrial DNA (mtDNA). In March 2010, discovery was announced of bone fragments of a juvenile that lived about 41,000 years ago found in Denisova Cave (Altai Krai, Russia), a region also inhabited at about the same time by Neanderthals and modern humans.[1][2] The mtDNA of the Denisova hominin is distinct from the mtDNAs of Neanderthals and modern humans.[3] In December 2010, an international team of scientists determined the sequence from the nuclear genome of this group (known as the Denisovans) from this finger bone. According to their analysis, this group shares a common origin with the Neanderthals and interbred with the ancestors of modern Melanesians.[4]

Contents

Anatomy and lineage

Little is known of the precise anatomical features of the Denisovans since the only physical remains discovered thus far are the finger bone from which only mitochondrial genetic material was gathered. A tooth found in Denisova Cave carries a mtDNA very similar to that of the finger bone and shares no derived morphological features with Neanderthal or modern humans.[5] The Siberian bone's mtDNA differs from that of modern humans by 385 bases (nucleotides) in the mtDNA strand out of approximately 16,500, whereas the difference between modern humans and Neanderthals is around 202 bases. In contrast, the difference between chimpanzees and modern humans is approximately 1,462 mtDNA base pairs. Analysis of the specimen's genome shows it to be due to a common branch of ancestors with Neanderthal lineage, but, after they diverged from one another, Denisovans and Neanderthals had largely separated population histories.[1][4][5][6]

Discovery

In 2008, Russian archeologists working at the site of Denisova Cave in the Altai Mountains of Siberia uncovered a small bone fragment from the fifth finger of a juvenile hominin, dubbed the "X-woman" (referring to the maternal descent of mitochondrial DNA[6]), or the Denisova hominin. Artifacts, including a bracelet, excavated in the cave at the same level were carbon dated to around 40,000 BP.

A team of scientists led by Johannes Krause and Swedish biologist Svante Pääbo from the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, sequenced mtDNA extracted from the fragment. Because of the cool climate in the location of the Denisova Cave, the discovery benefited from DNA's ability to survive for longer periods at lower temperatures.[2] The analysis indicated that modern humans, Neanderthals, and the Denisova hominin last shared a common ancestor around 1 million years ago.[3] Some studies suggest that modern humans coexisted with Neanderthals in Europe, and the discovery raises the possibility that Neanderthals, modern humans and the Denisovan hominin may have co-existed.[citation needed]

The DNA analysis further indicated that this new hominin species was the result of an early migration out of Africa, distinct from the later out-of-Africa migrations associated with Neanderthals and modern humans, but also distinct from the earlier African exodus of Homo erectus.[3] Professor Chris Stringer, human origins researcher at London's Natural History Museum and one of the leading proponents of the recent single-origin hypothesis, remarked: "This new DNA work provides an entirely new way of looking at the still poorly understood evolution of humans in central and eastern Asia." Pääbo noted that the existence of this distant branch creates a much more complex picture of humankind during the Late Pleistocene.[6]

Later in 2010, a second paper from the Svante Pääbo group reported the prior discovery, in 2000, of a third upper molar from a young adult, dating from about the same time (the finger was from level 11 in the cave sequence, the tooth from level 11.1). The tooth differed in several aspects from those of Neanderthals while having archaic characteristics similar to the teeth of Homo erectus. They again performed mitochondrial DNA analysis on the tooth and found it to have a different but similar sequence to that of the finger bone, indicating a divergence time about 7,500 years before, and suggesting it belonged to a different individual from the same population.[5]

Nuclear genome analysis

In the same second 2010 paper, the authors report the isolation and sequencing of nuclear DNA from the Denisova finger bone. This specimen showed an unusual degree of DNA preservation and low level of contamination. They were able to achieve near-complete genomic sequencing, allowing a detailed comparison with Neanderthal and modern humans. From this analysis, they concluded that in spite of the apparent divergence of their mitochondrial sequence, the Denisova population along with Neanderthal shared a common branch from the lineage leading to modern African humans. The estimated time of divergence between Denisovans and Neanderthals is 640,000 years ago, and that between both these groups and modern Africans is 804,000 years ago. They suggest that the divergence of the Denisova mtDNA results either from the persistence of a lineage purged from the other branches of humanity through genetic drift or else an introgression from an older hominin lineage.[5]